Aging gauge

ABSTRACT

An aging gauge comprising a container having a fixed or a variable sized  t opening with a cap which can be opened to control the sublimation rate of a thermally sublimational material contained within the container. In use, the aging gauge is stored with an item to determine total heat the item is subjected to and also the maximum temperature to which the item has been exposed. The aging gauge container contains a thermally sublimational material such as naphthalene or similar material which has a low sublimation rate over the temperature range from about 70° F. to about 160° F. The aging products determined by analyses of a like item aged along with the aging gauge for which the sublimation amount is determined is employed to establish a calibration curve for future aging evaluation. The aging gauge is provided with a means for determining the maximum temperature exposure (i.e., a thermally indicating material which gives an irreversible color change, Thermocolor pigment). Because of the relationship of doubling reaction rates for increases of 10° C., equivalency of item used in accelerated aging evaluation can be obtained by referring to a calibration curve depicting storage temperature on the abscissa scale and multiplier on the ordinate scale.

DEDICATORY CLAUSE

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto us of any royalties thereon.

BACKGROUND OF THE INVENTION

Estimating the useful life of production materials or a combination ofthese material is necessary, and particularly even of a higher magnitudeof importance is the estimating of the effects of aging on solidpropellant materials of solid propellant rocket motors.

Presently, items such as solid propellant rocket motors are stored inthermal environments which have little or no control and are seldommonitored. However, when monitored, devices such as recordingthermometers are used. These thermometers can record the temperatureextremes and temperature-time profile, and by integrating under atemperature curve, the temperature and total heat effects can bemeasured. Such equipment is costly, requires a constant energy source tooperate, and monitors a general area and not a specific item in total.

Of particular interest and benefit would be a low cost item which canmonitor specific items. For example, an aging device which can beattached directly to a specific item would be attractive. An agingdevice which can be attached directly to a specific item, and an agingdevice which is passive, would be particularly attractive since itrequires no operating energy source.

Therefore, an object of this invention is to provide an aging devicewhich requires no operating energy source.

Another object of this invention is to provide an aging device which canbe attached directly to individual items for which aging data orinformation is desired.

Still another object of this invention is to provide an aging devicewhich can provide an irreversible color change which can indicate themaximum temperature to which the item is exposed.

An additional object of this invention is to provide an aging devicewhich can be employed with an item to correlate natural aging or anaccelerated aging based on calibrated data obtained on the aging deviceand the item with which the aging device is stored.

SUMMARY OF THE INVENTION

The aging gauge of this invention comprises a container with a caphaving a fixed opening or a variable sized vent which can be opened tocontrol the sublimation rate of a thermally sublimational materialcontained within the container. In use, the aging gauge is stored withan item to determine total heat to which the item has been subjected andalso the maximum temperature to which the item has been exposed. Theaging gauge container contains a thermally sublimational material suchas naphthalene or similar material which has a low sublimation rate overthe temperature range from about 70° F. to about 160° F. The amount ofsublimation over a time period is calibrated from comparison valuesalong with the amount of aging products found to have taken place with apropellant composition subjected to a like temperature range. Thecalibration data serves to correlate propellant aging with the amount ofsublimation from the aging gauge. This correlation gives a temperatureversus time relationship which serves to predict the life of an itemsuch as a propellant composition where chemical reaction rates areknown, and their effects are known with respect to temperature and totalheat exposure. The aging gauge is additionally provided with a series ofthermocolor pigments which are formulated to yield irreversible colorchanges at a maximum temperature to which it is subjected. Thesepigments can be placed on the inside or outside of the aging gauge. Thecontainer for the aging gauge can be metal, glass, or plastic; and whenclear glass or plastic, the quantity of low temperature sublimationalmaterial remaining can be observed. Likewise, the thermocolor pigmentcan be observed for irreversible color changes when placed on the insideof the container when the container is transparent.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 depicts a schematic view of an aging gauge of this invention.

FIG. 2 depicts a schematic view of an unitary container containing athermally sublimational compound, a temperature indicator, and a unitarycontainer which has a fixed vent opening for yielding a constantsublimational reaction rate for the aging gauge.

FIG. 3 depicts an accelerated aging evaluation curve for agingtemperatures of 70° F., 145° F., and 165° F.

FIG. 4 is a calibration curve depicting grams/day loss of naphthalenethrough a fixed sized vent hole over a temperature range of 70° F.-160°F.

FIG. 5 depicts a multiplier equivalent age referenced to 70° F. andcalculated from the naphthalene loss through a fixed size vent hole.

FIG. 6 depicts operational limits for temperature range 70° F.-160° F.versus time scale of 10 years.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The aging gauge for determining the maximum temperature to which an itemhas been exposed is one of the features derived from the aging gauge ofthis invention.

Another feature and perhaps the most important one is a means formeasurement of the total heat, based on the occurrence of certain eventsto which an item has been subjected.

With reference to the drawing, FIG. 1 depicts an aging gauge 10comprised of a container 12 which can be made of metal, glass, orplastic. The container 12 has a cap 14 with a variable sized opening 16which serves as a vent hole. A thermally sublimational material(naphthalene) 18 is loaded into the container and then sealed. The aginggauge device is precisely weighed before and after the sublimationalmaterial is loaded into the container.

FIG. 2 depicts another embodiment of an aging gauge 20 comprised of aglass or plastic container 22 and having a removable plug 24 for a fixedvent opening 26. An irreversible color changing pigment 28 serves toindicate the maximum temperature exposure reached. The sublimationalmaterial 29 is shown inside container 22 in a predetermined amount whichcan be measured with time elapse by observing the level or by weightloss determination.

After the aging gauge or device is loaded and precisely weighed, it isthen sealed. When put in use by storing with an item for agingevaluation, the vent of the aging gauge is opened and when the vent isopened, the thermally sublimational material sublimes at a ratedepending on temperature. The weight changes can be measured with timeor in the case of an aging gauqe with a glass or transparent containerthe level of thermally sublimational material can be measured orobserved to determine the amount sublimated. Knowing the agingcharacteristics of the stored item such as a rocket motor and thesublimation quantity based on exposure to a certain quantity of heat,then with the two items stored together, a correlation of the aginggauge and the aging characteristics can be closely related whereby thechange due to a known sublimational reaction translates to a meaningfulaging gauge for the stored rocket motor.

After the sublimational chemical is loaded, either a precise weight orvolume is recorded. In the case of volume comparison, the liner lengthmay be used to measure rate regression if all but one surface isinhibited. Thus, by storing gauges at several elevated temperatures, acalibration curve can be obtained giving a temperature versus timerelationship for any given temperature with a given sized vent hole.

A thermally indicating material is added to the side of the aging gaugecontainer so after being exposed to a maximum temperature, airreversible color change takes place at a given maximum temperature.The described material in the form of irreversible color changingpigment is sold under the trademark Thermocolor of BASF-Wyandotte,Wyandotte, Mich. 48192.

As noted by further reference to FIG. 1, thermally indicating material19 having a wide range of color changes can be selected to cover fromambient temperature to an accelerated aging temperature (from about 70°F. to about 160° F.) to determine the highest temperature to which astored item has been subjected. If a number of aging gauges with a widerange of thermally indicating materials are placed on rocket motors atselected locations, then hot spots can be detected by reference to thecolor changes irreversibly made. This type information can be of valuein predicting what reactions or changes have taken place in an area suchas in the propellant grain, or in a composite insulation material, etc.This correlation with the history of the sublimational portion of theaging gauge provides information which by further evaluation andinterpretation is indicative of whether the item, such as a rocketmotor, has deteriorated to a point at which it should be pulled fromnormal, reliable service.

In further reference to FIG. 3, it is noted that the equivalency of timeused in accelerated aging evaluation is based on a long establishedreaction phenomenon; that is, with a temperature increase of each 10° C.or about 18° F., the reaction rate doubles. Thus, the multiplier valueon the ordinate scale projects a reaction rate of 1.0 at 100° F.;however, when the storage temperature is raised to about 154° F., thereaction rate is about 8 times what it would be at a storage temperatureof about 100° F. The ambient temperature of 70° F. when raised to about160° F. results in the reaction rate being increased from about 1.0 at70° F. to about 35 times the rate when elevated to 160° F. Beinginterpreted further in terms of one day at 70° F. the aging equivalentbased on reaction products formed at 160° F. translate to about 41minutes. Additional equivalent accelerated aging values relationshipsare set forth in Table I below for ambient, 145° F., and 165° F.compared with ambient temperature of 70° F.

                  TABLE I                                                         ______________________________________                                        At 145° F.                                                                            At 70° F.                                                                            At 165° F.                                ______________________________________                                        1 hr 20 min                                                                             =        1 Day    =      41 min                                     9 hr 20 min                                                                             =        1 Week   =      4 hr 48 min                                40 hr     =        1 Month  =      201/2 hr                                   10.1 Days =        6 Months =      51/4 Days                                  201/4 Days                                                                              =        1 Year   =      101/2 Days                                 401/2 Days                                                                              =        2 Years  =      20.8 Days                                  81 Days   =        5 Years  =      413/4 Days                                 1621/4 Days                                                                             =        10 Years =      831/2 Days                                 ______________________________________                                    

In further reference to FIG. 4 which is a calibration curve depictinggrams/day loss of naphthalene through a 0.325 inch diameter ventopening, and in further reference to FIG. 5 which is a multiplierequivalent age referenced to 70° F. and calculated for an extended time,it is concluded that the rate of loss of 70° F. and 160° F. follows therule of thumb which depicts a doubling of the reaction rate for every10° C. or 18° F. which also coincides with the accelerated aging datashown hereinabove and depicted in FIG. 3.

In further reference to FIG. 6 which depicts the operational andnon-operational time limits based on a deviation from 70° F. and astorage life of 10 years, the remaining time of useful life can bepredicted as further disclosed hereinbelow.

In evaluating the aging gauge vent hole diameter, it has been found thatthe vent hole must be greater than 0.125 inches diameter to be useful(i.e., sublimates without restricting the opening). An aging gauge witha 0.325 inch diameter vent hole looses naphthalene at a rate as follows:

0.00134 grams/day at 70° F.

0.04245 grams/day at 160° F.

The calculated loss per year is 0.475 grams or 4.75 grams per 10 years.The rate of loss at 160° F. when compared to 70° F. is about a 32 timesincrease which follows the rule of thumb (for every 10° C. increase intemperature, a chemical reaction rate doubles) and which also coincideswith the operational temperature curve, FIG. 3.

The following working example indicates the usefulness of the aginggauge. An aging gauge with a vent hole of 0.325 is placed next to astored item. The gauge contains 5.0 grams of naphthalene. In three yearsthe loss of naphthalene is three grams. Since the rate loss at 70° F. is0.00134 grams/day the equivalent aging time is 3.0 grams/0.4252 year or6.31 years. Conclusion: The item has experienced a thermal environmentto give an equivalent age of 6.31 years at 70° F. The item then has only3.69 years storage life left, if the remaining time is at 70° F.

Aging in a thermal environment is assumed to occur at an exponentialrate which doubles every 10° C. The weight loss of material in the aginggauge is at about the same exponential rate with temperature increases;therefore, the weight loss can be correlated to an equivalent age.Storage life is referenced to 70° F. and can be projected for storagetimes at elevated temperature.

We claim:
 1. An aging gauge for a component of a solid propellant rocketmotor wherein said component when aged for predetermined time periodsand predetermined temperatures forms aging prducts which can bedetermined by analyses methods, said aging gauge comprising:(i) acontainer for containing a thermally sublimational material; (ii) anopening having a removable seal in said container which said opening isnormally selaed after loading said thermally sublimational material intosaid container and when said aging gauge is placed into service saidseal is effectively removed, said opening then serving as a vent for ameasurable sublimination reaction; (iii) a predetermined amount of athermally sublimational material loaded into said container, saidthermally sublimational material characterized by having a lowsublimation rates over the temperature range from about 70° F. to about160° F. and by having a measurable loss amount of said thermallysublimational material after an elapsed aging time period for comparisonwith the amount of aging products found in a component of a solidpropellant rocket moter aged with said aging gauge for a like elapsedtime period to establish calibration data for said aging gauge wherebyan aging gauge of a like construction serves to determine the total heatthat a component of a solid propelleant rocket motor has been subjectedto and which serves to determine the aging products expected to bepresented from a comparison of the sublimation amounts with the analysesdeermined aging products present when a calibration curve is establishedfor said aging gauge; and, (iv) means affixed to said aging gauge fordetermining the maximum temperature to which said aging gauge issubjected to during aging service.
 2. The aging gauge as defined inclaim 1 wherein said thermally sublimational material placed in saidcontainer is naphthalene.
 3. The aging gauge as defined in claim 2wherein said container is a unitary container and wherein said openingis a predetermined fixed vent opening size for yielding a constantsublimation rate.
 4. The aging gauge as defined by claim 3 wherein meansaffixed to said aging gauge for determining the maximum temperature towhich said aging gauge is subjected are in the form of irreversiblecolor changing pigments covering said temperature range of said aginggauge.
 5. The aging gauge as defined in claim 2 wherein said containeris provided with a removable cap having an opening whereby said openingachieves a variable sublimation rate proportional to said opening andthe temperature to which said aging gauge is subjected.
 6. The aginggauge as defined by claim 5 wherein means affixed to said aging gaugefor determining the maximum temperture to which said aging gauge issubjected are in the form of irreversible color changing pigmentscovering said temperature range of said aging gauge.